A fuel cell is a device that converts chemical energy from a fuel into electricity through chemical reactions, in contrast to combustion of the fuel in an exothermic reaction to produce heat. While many different types of fuel cells have been developed, they all have the same general components and function in the same general manner. A fuel cell consists of three components (other than the fuel and the oxidant), typically layered or sandwiched together: the anode, the cathode and an electrolyte in between. Two chemical reactions occur at the interfaces of the three different components. The net result is that fuel is consumed, water and carbon dioxide are created, and an electric current is produced which can be used to power electrical devices. Fuel cells differ from batteries in that they require a supply of fuel and oxidant to operate, but they can produce electricity so long as fuel and oxidant are supplied.
At the anode, a catalyst oxidizes the fuel, such as hydrogen or a hydrocarbon, thereby producing a positively charged ion and a negatively charged electron. This is commonly referred to as the “oxidation” reaction. For a hydrocarbon fuel, the oxidation also produces carbon dioxide as a waste product. The electrolyte is a substance specifically chosen so ions can pass through it, but the electrons cannot. The freed electrons travel through a power circuit creating the electric current. The ions travel through the electrolyte to the cathode. Upon reaching the cathode, the ions are reunited with the electrons that created the electric current, and the two react with a third chemical, an oxidant such as oxygen, to create water as a waste product. This is commonly referred to as the “reduction” reaction.
In order to effect the oxidation reaction at the anode and the reduction reaction at the cathode, catalysts are used at the anode and the cathode which catalyze the respective reactions.
Accordingly, the most important design features in a fuel cell are:
(1) The electrolyte substance or structure. The electrolyte substance usually defines the type of fuel cell, such as a proton exchange membrane fuel cell (PEMFC). In solid membrane designs, the membrane electrolyte is commonly assembled with an anode and a cathode and used as a membrane electrode assembly, or MEA.
(2) The type of fuel used in the fuel cell. The most common fuel is hydrogen, but hydrocarbons such as natural gas, formic acid and methanol may also be used.
(3) The anode catalyst, which breaks down the fuel into electrons and ions. The anode catalyst is often made up of platinum.
(4) The cathode catalyst, which turns the ions into the waste products, such as water, in the reduction reaction with an oxidant and the freed electrons. The cathode catalyst is often made up of platinum or nickel.
A typical fuel cell produces a voltage from 0.6 V to 0.7 V at full rated load. To deliver more energy than a single fuel cell, the fuel cells are combined in series and parallel circuits, where series yields higher voltage, and parallel allows a higher current to be supplied. Such a design is called a fuel cell stack. In addition, the cell surface area of a fuel cell can be increased to provide more current from each cell.
As referred to above, the hydrogen-oxygen PEMFC is the prime example of a fuel cell design. As in its name, the fuel is hydrogen and the oxidant is oxygen. The PEMFC utilizes a solid polymer electrolyte membrane (PEM) between the anode and the cathode. The PEM allows only positively charged ions to pass through it to the cathode, while negatively charged electrons are blocked and must travel along an external power circuit.
On the anode side, hydrogen fuel is supplied and diffuses to the anode catalyst which oxidizes the hydrogen into protons and freed electrons. The protons pass through the PEM to the cathode, while the electrons are forced to travel along an external power circuit because the PEM is electrically insulating.
At the cathode, the electrons (after passing through the power circuit) and positively charged ions react with the oxygen molecules of the supplied oxidant to form water as the waste product.
The fuels used in fuel cells have relatively high energy density compared to other power sources. For example methanol has an energy density of 4900 Wh/L and formic acid has an energy density of 2104 Wh/L. Due to the very high energy density of their fuels, fuel cells are a very attractive power source for both macro and micro systems. In particular, miniature fuel cells have broad applications in portable devices such as laptop computers, cellular phones and global positioning systems (GPS).
However, fuel cells face a few important challenges in order to contend as viable candidates for micro power sources. Some of these challenges include “packaging penalty” (explained below), fuel crossover, fuel delivery, water management, and waste product removal.
One of the main challenges in miniaturizing fuel cells below a centimeter has been the need of ancillary components such as a fuel pump (to maintain the required flow of fuel) and a gas separator to build a complete fuel cell system. Because these parts are not easily scaled down, they take up increasingly more volume relative to the fuel in a miniaturized system—a critical issue known as the “packaging penalty” referred to above.
To address some of these issues, U.S. Pat. No. 7,976,286 describes a self-circulating fuel cell, which eliminates several of the ancillary parts, such as the liquid pump for fuel delivery and the phase separator for CO2 bubble removal. The contents of this patent is expressly incorporated by reference herein in its entirety. The device described in this patent utilizes an embedded bubble pumping mechanism, which uses directional bubble growth and a selective gas venting, to flow fuel within an anodic microchannel. An electrode-PEM-electrode sandwich, or membrane electrode assembly (MEA), separates the anodic microchannel containing a fuel from a cathodic microchannel containing an oxidant. The CO2 bubbles generated by the oxidation of fuel within the anodic microchannel are used to pump fuel through the anodic microchannel. The CO2 bubbles are then removed from the anodic microchannel through a non-wetting venting membrane, thereby obviating the need for a gas separator because the gas bubbles simply diffuse through the venting membrane to the outside environment. Thus, without using any discrete pump component or gas separator, the fuel is actively circulated through the anodic microchannel and the CO2 waste is removed, thereby maintaining the fuel concentration. In contrast, passive fuel cells rely only on fuel diffusion to the anode, with the inevitable tendency to develop a depletion zone over time.
Although the self-pumping mechanism of U.S. Pat. No. 7,976,286 greatly simplified the anode side of the fuel cell, the oxygen source for the cathode side remains a bulky attachment in the form of a pressurized oxygen tank, a fan or microchannels filled with actively-pumped, oxygen-saturated electrolyte solution. Furthermore, the two chamber configuration, separated by an MEA, is closely associated with many problems of fuel cell systems, including fuel crossover, cathode flooding, anode dry out, and difficulty in miniaturization.
Accordingly, there is a need for an improved fuel cell design which overcomes some of the problems of previous fuel cells, and preferably is capable of miniaturization below centimeters, and/or for use in micro-electromechanical system (MEMS) applications.